Semiconductor elements such as a transistor and a diode are used in circuits for a switching power supply and an inverter. These semiconductor elements require a high breakdown voltage and a low on-resistance. The relationship between the breakdown voltage and the on-resistance is a trade-off relationship determined by a semiconductor material used for the semiconductor elements.
With the progress of technological development so far, the semiconductor element realizes the low on-resistance near the limit of silicon which is the major semiconductor material. To further increase the breakdown voltage or further reduce the on-resistance, there is a need to change the semiconductor material.
Nitride semiconductors such as gallium nitride and aluminum gallium nitride have a bandgap larger than that of silicon. By using the nitride semiconductor, the trade-off relationship determined by the semiconductor material can be improved and the dramatic high breakdown voltage or low on-resistance can be realized.
In the transistor using the nitride semiconductor, a fluctuation in threshold voltage may occur due to application of voltage stress. It is expected to realize a transistor capable of realizing high reliability by suppressing the fluctuation in the threshold voltage.